1. Field of the Invention
The invention relates to a process for manufacturing cylindrical hollow bodies with a circular cross section using solid rough material made of corrosion-resistant martensitic chromium steel, particularly rings to be subjected to high mechanical stress at least at sections of the cylinder zones near the surface. The invention also relates to the hollow bodies made by this process.
2. Discussion of Background Information
Ring-shaped machine and tool components, such as circular shear blades, roller bearing rings and so forth, may be subject to high mechanical stresses at the cylinder zones near the surface. A high load capacity for surface pressure, high wear resistance, a good level of toughness and high shearing strength of the material are all required in the component regardless of the direction. For special areas of application, in addition to the mechanical properties, the resistance of the material to corrosion is also very important. This property profile can be achieved synergetically by means of alloying techniques.
Tubes can be produced by various processes as rough material for cylindrical hollow bodies or rings, which have a high mechanical load-bearing capacity in all directions at the cylinder surfaces and/or at the adjoining edges. The choice of a specific manufacturing process depends on its applicability for the material, the required product characteristics and/or its economic efficiency.
The highest material quality in highly alloyed rings or hollow bodies made of tubes, can be achieved when a casting or rough material block is formed by forging or rolling while reducing the cross-section by hot forming all round essentially perpendicular to its axis, thus stretching it lengthwise into a round bar. Next, a tubular bar is formed by turning the center or drilling, particularly deep hole boring, from which bar the rings are cut. In the warm forming process an intensive kneading of the material (alloy) occurs, making it possible to produce a material with isotropic characteristics. It also is possible to rough-work individual hollow bodies out of a forged or rolled steel rod, preferably by automatic turning or drilling, whereby, if necessary, center segregation spots can also be machined off. Hollow bodies produced by this method have a particularly high material quality. However, the manufacturing costs are high, the production process is complicated and the cutting waste is considerable.
The use of rolled tubes as the base material for the production of inner or outer rings for roller bearings is known. DE-A-19520833, for instance, shows a process by which is produced basically a continuous casting material of hypereutectoid chromium steel with a high degree of purity, fine carbide precipitations and a highly fine-grained microstructure and wherein the length for use is heated to forming temperature in the as-cast state and without heat treatment and fed into a tube production installation, preferably including a piercing press. In the piercing process, a state of stress is built up in the individual lengths to be formed, which shows as high a negative mean strain value as possible while minimizing shearing strain. As known from DE-C-19734563, both the state of strain during piercing to prevent the material from cracking and the establishment of a specific microstructure are important for securing a high quality of the roller bearing rings.
A perforator equipped as a skew-rolling mill, followed by at least one tube rolling mill can also be used to manufacture seamless tubes as starting material for the production of roller bearing rings made of steels customary for this purpose.
The conventional tube manufacturing processes mostly show a high level of economic efficiency, but they have in common the disadvantage that they cannot be used for highly alloyed tool steels, e.g., for corrosion-resistant martensitic chromium steels. In order to be corrosion-resistant, these kinds of steel have chromium contents of more than about 12 percent by weight and, optionally, are alloyed with molybdenum. For achieving the desired mechanical properties of the material upon heat treatment of the alloy, high carbon concentrations must also be provided for.
At forging temperature, highly alloyed heat-treatable steels usually exhibit material characteristics that preclude perforation and tube rolling. Particularly when making and expanding the perforation of employed material by mandrels or similar tools, cracks form in the material as a result of high tensile and shearing stresses, making it impossible to manufacture pipes of the desired quality.